1. Technical Field
The present disclosure relates to a microelectromechanical sensor with a non-conductive sensing mass and to a method of sensing through a microelectromechanical sensor.
2. Description of the Related Art
Known to the art are microelectromechanical sensors of various types, which exploit the relative displacements of a movable mass with respect to a supporting structure. Sensors of this type are spreading to an ever-increasing extent in numerous apparatuses and may comprise, for example, accelerometers, gyroscopes, and electro-acoustic transducers (microphones/loudspeakers).
The displacements of the movable mass are determined by variations of the quantity that is to be measured. In the case of an accelerometer, for example, forces applied to the supporting structure modify its state of motion and cause relative displacements of the movable mass. In gyroscopes, the movable mass, kept in controlled oscillation, displaces as a result of the Coriolis acceleration due to rotations of the supporting structure.
In electro-acoustic transducers, the movable mass is in the form of a membrane that undergoes deformation in response to incident acoustic waves.
Consequently, from the amount of the displacement of the movable mass, it is possible to derive the value of the quantity that has caused it.
In many sensors, the movable mass is capacitively coupled to the supporting structure, and the capacitive coupling is variable in proportion to the position of the movable mass itself. From the information on the capacitive coupling, which can be easily obtained at electrical terminals, the quantity to be measured is derived.
According to widely adopted solutions, the supporting structure and the movable mass have respective mutually facing conductive electrodes so as to form capacitors. The capacitances of the capacitors are determined by the distance between the electrodes of the supporting structure and the electrodes of the movable mass and hence depend upon the position of the latter. Between the electrodes generally air is present.
Notwithstanding the wide range of applications, there are, however, some aspects that limit the performance and, sometimes, the possibility of use of capacitive sensors of this type.
The most critical aspects, which are frequently in conflict, are in general the sensitivity and the linearity of the sensors. The sensitivity, defined as derivative of the capacitance with respect to the position in the sensing direction, basically depends upon the geometry of the sensor (surfaces of the electrodes and distance at rest) and upon the stiffness of the suspension elements that connect the movable mass to the supporting structure to enable elastic oscillations with respect to pre-determined degrees of freedom or else upon the stiffness of the membrane in the case of electro-acoustic transducers. In particular, stiffer springs or membranes enable displacements of modest proportions and, consequently, small capacitive variations. If, on the one hand, the linearity benefits from small displacements from a resting position, on the other hand, however, the sensitivity is limited and this results in lower accuracy and robustness to noise. Less stiff elastic connections and membranes are to the advantage of sensitivity, but reduce the linearity. In addition, the risk of impact between the movable parts and the fixed parts, which may even cause irreversible damage to the devices, increases.
A further limit derives from the need to provide electrical connections both for the fixed electrodes and for the movable electrodes. The architecture of the microelectromechanical sensors is frequently complex, and providing numerous electrical connections may prove problematic.
There has been proposed the use of sensors that exploit capacitors, which are provided on the surface of a substrate and are biased, and a movable mass, which is made of polymeric material (for example parylene) and is set at a variable distance from the substrate. The movable mass, according to the position with respect to the surface of the substrate, modifies differently the lines of field at the edge of the capacitors and, consequently, their capacitance.
This solution presents, however, limitations because polymers, and in particular parylene, are far from suitable to create complex microstructures, as in many cases would be necessary, instead. The flexibility is consequently poor and the possibilities of use are rather limited.